(i) Field of the Invention:
The present invention relates to an improved papermaker's pressure belt for extended nip presses for dewatering a fibrous web of paper.
(ii) Description of the Prior Art:
Conventional extended nip presses of two different types are illustrated in FIGS. 8A and 8B. The extended nip press of FIG. 8A comprises a press roll 1, a pressure shoe 2 having a concave arcuate surface facing the press roll and having about the same radius of curvature as the press roll 1, and an endless pressure belt 3. A wet fibrous web 5 is traveled between the pressure belt 3 and a felt 4 trained around the press roll 1, and the wet fibrous web 5 is squeezed by the pressure belt 3 forced towards the press roll 1 by the pressure shoe 2. The extended nip press of FIG. 8B is provided with an additional felt 4' trained along the pressure belt 3. In this extended nip press, a wet fibrous web 5 is sandwitched between.the two felts 4 and 4' for squeezing.
In the former extended nip press of FIG. 8A, one side of the pressure belt 3 is in direct contact with the wet fibrous web 5 and the other side is in direct contact with the pressure shoe 2, respectively. Accordingly, the surface of the pressure belt 3 which is in contact with the wet fibrous web 5 should be smooth, since the surface condition directly affects the finish of the surface of the fibrous web, while the other surface of the pressure belt 3 which is in contact with the pressure shoe 2 needs to be processed so that the lubricant supplied to the acting surface of the pressure shoe 2 is prevented from permeating the pressure belt 3. In order to provide a pressure belt with such properties, a conventional pressure belt comprises a belt base 3a composed of a woven fabric or the like, and coating layers 3b and 3b' of highly abrasion-resistant synthetic resin such as polyurethane resin formed on the both faces of the belt base 3a, respectively, as illustrated in FIG. 9. However, in a high-speed papermaking process as represented in FIG. 8A, a wet fibrous web 5 tends to stick to such a smooth coated layer, and hence a separation of the wet fibrous web 5 from such a pressure belt does not readily take place.
On the other hand, in the extended nip press of FIG. 8B, since the pressure belt 3 is forced indirectly towards a wet fibrous web 5 through the felt 4', the surface of the pressure belt 3 which faces the wet fibrous web 5 need not be smooth. Therefore, a coating layer 3b' needs to be formed only on one surface of a belt base 3a to be in contact with the pressure shoe as illustrated in FIG. 10. It is also possible to have a belt base coated with coating layers on both faces thereof and provided with draining grooves in the coating layer which faces the wet fibrous web for draining water squeezed from the wet fibrous web by the felt 4'. Such a pressure belt having grooved squeezing surface, when used in combination with a grooved press roll, improves the squeezing performance of the extended nip press. Furthermore, since the wet fibrous web is sandwitched between the upper and lower felt belts, it can be released easily from those upper and lower felt belts even in a high-speed papermaking operation. However, since the extended nip press of FIG. 8B is provided with two felt belts, it has the disadvantage that the papermaking machine employing this extended nip press is inevitably large in constructions and is expensive.